The exposure-control panel is the master control
for the ccd program. It controls exposure and readout
modes, as well as providing manual control of various operations.
Contents
- Overview
- Normal exposures and the exposure control-area
- The mode-control check-boxes
- The manual controls
- The file control-area
- The comment field
- The filter control-area
- Known bugs
- Version notes
Overview
The exposure-control panel is automatically opened at
start up. It's supposed to be impossible to close, miniaturize,
or hide this window while this application is active. If the
impossible somehow happens, the window may be brought back to the
front by selecting the tools
option, control...
sub-option from the main
menu (normally found in the upper-left corner of the display).
Normal exposures and the exposure control-area
To expose the CCD, first enter the exposure
time. This is done by entering the exposure time, in seconds, in the
text field immediately below the EXPOSE button. Note:
times will be rounded to the nearest tenth of a second (before release 3.00, times were truncated
to the second). Then, press the EXPOSE button, and the
CCD will be exposed and read out, with the resultant image displayed as
it is read in. During the exposure, a count-down/count-up timer will be
prominently displayed. (The time shown is only a rough number, for the
operator's information; the actual timing is much more accurate.) If,
for some reason, you need to stop the exposure, press the STOP
EXPOSURE button in the count-down/count-up timer window. If the
READOUT check-box is set, the CCD will then be read
out.
Note: the timing of the exposure is believed to be more accurate than the uncertainties caused by the shutter mechanism.
Exposure control options
These options affect the way that the system behaves when an exposure
is taken, using the EXPOSE button
They have no effect when using the manual controls.
Normally, one would want to flush the CCD
prior to exposing. So the default is to perform flushing prior to
opening the shutter. If that is not desired, unset the FLUSH
check-box. (This would be used for the second to nth exposures on
a focus frame.) This check-box should not be confused with the
Flush button.
Users would likely need to perform dark exposures and focus frames. These require changes in
the default exposure parameters. To select a normal exposure, in which
the shutter will open at the beginning of the exposure and close at the
end, check the SHUTTER check-box. To perform a dark
frame exposure, uncheck the box.
Next, determine if you want to read the CCD out
immediately after exposing. This is the default, and so the
READOUT box is normally checked. If immediate readout is
not desired, uncheck the box. This would be used for the first to nth
exposures on a focus frame.
The LOOP check-box will cause
the entire sequence of events to repeat after the exposure is
complete, just as if the user pressed expose. Warning:
this should be carefully monitored or should be used with the SAVE check-box unset, as this will
cause the disk to run out of storage very quickly.
The mode-control check-boxes
The mode-control check-boxes (LOCAL, CHECK, ABM,
TCS, and SLOW) control the read-out and operation of the
CCD under all conditions. They have effect under all conditions,
whether the CCD is read out manually, using the Read button, or read out automatically
using the the EXPOSE button with the
READ check-box enabled.
The LOCAL check-box enables a
test function which generates a dummy test pattern in the NeXT DSP.
This test function is used instead of the normal data read
out. It is used to test the software for memory/DMA/throughput
problems. The external interface and CCD Controller are not used.
Note that no data will be read out from the CCD while this
function is enabled. This button should never be checked during
normal operation.
The CHECK check-box enables an
automatic test to verify that the incoming image data matches the
test pattern. It should only be used for diagnostics, and produces
error messages and slows the readout if used with anything but the
appropriate test pattern generator.
The ABM check-box (dynamic
anti-blooming) is currently non-functional. It will be implemented
in a later release.
The TCS check-box enables the
software to read information from the telescope control system.
This option is currently disabled by default, because it
has not been fully tested. The telescope control system is queried
for various parameters, which are stored in the fits file. When
the button is changed to the checked state, the TCS-control
panel is brought up. You must enter a TCS host (a computer that
is connected to the serial input line of the TCS.) At the UH-88-inch
telescope, this computer is "tycho".
When the SLOW check-box is set (the
default value), readout does not take place until the data link has
been checked. This improves the reliability of the system, so should
normally be left undisturbed. Unsetting this option reduces the
duration of the expose / readout cycle by one or two seconds, so is
useful to get a very fast readout rate. You
may want to unset this option if you taking a series of short
exposures, with small frame sizes. (Note: this
feature was added in release 3.04
as the "SAFE" option, and was renamed to "SLOW" in release 3.05.)
The manual controls
The manual controls (Read, Flush, and
Idle), are used to manually perform various CCD-control
operations. Note that some of these operations may be performed
automatically by the EXPOSE
button.
To read out the CCD manually, press the
Read button. Readouts are normally accomplished
automatically after an exposure by checking the Readout
box next to the EXPOSE button.
To flush the CCD (i.e., dump the charge
rapidly), press the Flush button. If the CCD has been
saturated, several flushes may be necessary to clear the excess
charge. If the FLUSH check-box (next
to the EXPOSE button) is enabled,
then the CCD will automatically be flushed before exposure.
The Idle button is used to put
the CCD in a continuous clocking mode. It is generally used only
for testing purposes. Note that the Idle button
remains depressed after it is pressed; it returns to its normal
mode whenever any command stops the idling. This is intended as
a visual reminder that the system is not integrating, not
as an absolute indicator of the current state; unfortunately there
are several things that can modify the idle state. In particular,
upon entry to the program, the idle state is unknown and is assumed
to be non-idling. Any command that is issued to the remote DSP by
any other program will stop idling but will not affect the state
of the button. Pressing the Idle button when it is
in a depressed state will stop idling.
The file control-area
The file control-area is used to control the name of the output
data file. The SAVE check-box
determines whether data is saved to a permanent file or to a
temporary file. When the SAVE check-box is checked,
the Read (or EXPOSE with READOUT) commands cause the data
to be saved to a file that is determined by the name
and number fields. The number field is
incremented after each readout. If the SAVE button
is not checked, then data is always saved to a file named
"junk.fits"; there is no protection against over-writing
this file. Once this file is overwritten, all data that was
previously in it is lost. If SAVE mode is disabled
and LOOP mode is enabled, only
the last frame is saved to disk. In non-save mode, the SAVE
button is red, to warn the user that data may be lost. There
is no other warning or other indication that the SAVE
mode is disabled, so this option should be used with extreme
caution.
The comment field
The Comment field permits the user to enter a text value
that will be stored as a comment in the data file.
The filter control-area
The filter control-area is intended to provide a way for the user
to control the filter wheel from within the ccd program.
Currently, nothing is implemented; however, the user may enter a
text value that will be stored within the data file, for informational
purposes.
Known bugs
The state of the idle button is supposed to indicate
whether idle mode is active or not. Unfortunately, it is not always
possible to know whether idle mode is active. In particular, if
another program does a download or any other action, idle mode is
disabled, but the button will remain in its on state.
Version notes
Before release 3.04,
"junk.fits" was written every time that an exposure was
made in loop + no-save mode. This was changed to write the file only
on the last exposure. This was done to increase the repeat rate for
small frames.
With release 3.04 the
SAFE button was added. This was renamed to
SLOW with release
3.05. When this is disabled, the data link is not checked before
beginning a long exposure. This makes it possible to increase the
repeat rate for short exposures.
The ABM and TCS buttons were added to the control panel with release 3.00. Also, the
LOCAL and CHECK check-boxes were moved from
the setup panel to the control panel.
Before release 2.06, the interruptible expose mode would occasionally cause the program to hang. It is believed that this problem has been solved.
Starting with version 2.04, exposure timing was derived entirely within the NeXT, via a timing loop running in the NeXT DSP. Versions prior to 2.04 used a timing loop which was resident in the CCD controller. That is still available, if this version proves to be not quite as accurate.
This document was written by Hubert Yamada (yamada@ifa.hawaii.edu) and Kevin Jim (kjim@terrasys.com).
